Styrene-based polymer resins have been conventionally used for various industrial materials such as electric appliances, sundries, cushioning materials, heat insulating materials, food containers and the like, because of their superior moldability, processability and well-balanced resin performances. Recently, they have been used as labels for food packaging and food containers by molding to a sheet or a film or by subjecting them to a further secondary processing.
In these fields of applications, a sheet or a film is generally required to have a smooth surface, transparency, less unevenness in thickness and superior strength characteristics. In general, styrene-based polymer resins are not practical in strength as a non-stretched sheet or film. However, it can be converted, by stretching, to a tough and stiff sheet or film with superior transparency, surface gloss and toughness. For this purpose, there are employed measures such as sheeting or stretching at a low temperature, and if necessary, further secondary processing at a low temperature.
However, a polystyrene resin as a typical styrene-based polymer resin is inferior in moldability to sheet or film, because of its relatively high softening point as well as its rigid and brittle properties. More specifically, in a sheeting or stretching process and also in a secondary processing at a low temperature, a polystyrene resin is inferior in processability and has problems such that uneven thickness is caused by poor fluidity and the resulting sheet is likely to be broken. Therefore, it is very difficult to obtain a sheet or a film having superior strength characteristics with a high stretching ratio.
For these fields of applications, a copolymer resin consisting of styrenic monomer and acrylate ester monomer or methacrylate ester monomer (hereinafter abbreviated as (meth)acrylate ester monomer) has been already known as a resin improved in the weak points of the polystyrene resin.
By copolymerizing styrene with a (meth)acrylate ester, the softening point can be adjusted, and the weak points of the polystyrene resin, i.e. rigidity and brittleness, can be also improved. Furthermore, a sheet or a film obtained by forming and processing the styrene-(meth)acrylate ester copolymer resin is superior also in strength characteristics.
More specifically, Reference 1 mentioned below discloses that a resin composition comprising a copolymer composed of a vinyl aromatic hydrocarbon and an aliphatic unsaturated carboxylic acid derivative and the like (including (meth)acrylate ester monomer); and a block copolymer of styrene-conjugated diene (hereinafter abbreviated as a styrene-based block copolymer), is superior in processability of cold draw, stretching characteristics and crack resistance of film.
In addition, Reference 2 mentioned below discloses that a low temperature shrinkable film, obtained by uniaxially stretching a resin composition comprising a copolymer composed of a vinyl aromatic hydrocarbon and an aliphatic unsaturated carboxylic acid derivative and the like and a styrene-based block copolymer, is superior in low temperature shrink performance, low temperature shrink stress, stiffness, transparency, crack resistance characteristics, dimensional stability and the like.
Further, Reference 3 mentioned below discloses that a sheet and a film of a resin composition comprising a copolymer of styrene and n-butyl acrylate and a styrene-based block copolymer with specified structure or characteristics has well-balanced physical properties.
However, the styrene-based polymer resin used for these polymer compositions was a composition that had problems in thermal and processing stabilities depending on conditions used, although it was superior in a balance of physical properties of a sheet or a film. A specific problem relating to thermal and processing stabilities includes a problem of resin rework.
With increasing consciousness of cost reduction and environmental problems in recent years, the reduction of industrial waste has been strongly required. Consequently, the rework of molding chip and the like generated in molding and processing processes has become inevitable. Therefore, a resin with processing stability durable to the rework is strongly desired. Here, “rework” means recycling of poorly molded articles as well as trimmed edges, broken resin chips and the like in the sheet molding. In view of this point, the processing stability of a conventional styrene-based polymer resin was not necessarily sufficient. In particular, in an application to transparent sheets and films that is a target of the present invention, coloring occurred by resin deterioration, foreign matters and uneven flow are very conspicuous, and can be fatal drawback of the rework of resin. Therefore, a technique such as blending a small amount of selected rework resin into a virgin resin is currently employed as a measure.
It has been heretofore conceived that the problems of thermal and processing stabilities of a styrene-based polymer resin is mainly caused by cyclic styrene oligomers present in the resin. More specifically, cyclic oligomers present in a styrene-based polymer resin thermally decompose during processing to generate a radical (see, for example, Reference 4 mentioned below). Deterioration is mainly caused by scission or crosslinking of polymer chains induced by the radical generated.
As a measure to solve this problem, the use of a polystyrene resin obtained by anion polymerization has been proposed. It is said that a polystyrene resin obtained by anion polymerization does not contain the cyclic styrene oligomers and thus has remarkably superior stability (also see Reference 4 mentioned below). However, it has not yet been industrially realized probably because of cost problem.
In view of less oligomers content, a styrene-based polymer resin obtained by suspension polymerization method is also known.
For example, a styrene-based polymer resin, which has a total weight of a monomer, a dimer, a trimer and a solvent being not higher than 0.8% by weight, a unit composition being 80 to 99.5% by mole of styrenic monomer and 0.5 to 20% by mole of (meth)acrylate ester monomer and a limited solution viscosity, has a wide range of molding conditions and well-balanced moldability and strength. As a method for manufacturing said styrene-based polymer resin, it is known that any of mass polymerization method, solution polymerization method, suspension polymerization method and emulsion polymerization method can be used (see Reference 5 mentioned below). However, the above prior art references do not disclose any effect achieved by specifying the structure of a styrene-based trimer or any method therefor.
Reference 6 mentioned below discloses that a styrene-based resin obtained by a suspension polymerization at a low temperature contains not higher than 400 ppm of a styrene dimer and a styrene trimer. Reference 6 also discloses the copolymerization of a (meth)acrylate ester in an amount of less than 50% by weight and teaches the use of the obtained styrene-based polymer resin for a food packaging application by utilizing low elution of oligomer from the copolymer.
Furthermore, Reference 7 mentioned below discloses that a styrene-based copolymer composed of 50 to 80% by weight of styrenic monomer and 20 to 50% by weight of (meth)acrylate ester monomer, with unreacted monomer being not higher than 1000 ppm and the total weight of a styrene dimer and a styrene trimer being not higher than 1000 ppm, is superior in productivity in processing and appearance of molded article, has less odor and thus is suitable in an application such as food packaging containers. A manufacturing method disclosed in the Example of Reference 7 is limited to a suspension radical polymerization method.
However, the suspension polymerization method is usually limited to a batch process and the use of a dispersing agent is not avoidable in the polymerization step. Probably because of this reason, the styrene-based polymer resin obtained by a suspension polymerization method had problems such as coloring of resins due to processing history such as rework, easy whitening due to water absorption, or a disadvantage in cost.                Reference 1: JP-A-59-221348        Reference 2: JP-A-61-025819        Reference 3: JP-A-2001-002870        Reference 4: JP-A-09-111073        Reference 5: JP-A-04-239511        Reference 6: JP-A-2000-159920        Reference 7: JP-A-2001-026619        